A dynamic run-time environment for a language such as JAVA™ is responsible for managing memory for objects that are created and destroyed during the execution of a program. An object is an entity that encapsulates data and, in some languages, operations associated with the object. Since the encapsulated data is stored in memory, objects are associated with particular regions of memory that are allocated and deallocated by the dynamic run-time environment.
The state of a program, or “program state,” is the set of the objects and the references between the objects that exist at a specific point in time during the execution of the program. A “reference” is used by a run-time environment to identify and ultimately access the region of memory for storing the data of the object. Typically, references between objects in a run-time environment are encoded using machine pointers. A machine pointer is a native object that contains the address of the object in the main memory, which can be a real memory address or, more commonly, a virtual address on a machine that implements a virtual memory system. Since machine pointers are closely coupled to the underlying hardware and firmware of a computer system, machine pointers have high performance and, hence, are a popular implementation for references.
In a run-time environment, however, program state with machine-specific references such as machine pointers is sometimes disadvantageous. For example, it may be desirable to store the program state on disk or another secondary storage medium and restore the stored program state to main memory. Some run-time environments, in fact, are designed to use the same program state on different types of machines. For instance, such run-time environments provide load-balancing and crash recovery functions by transferring the execution of a program from one machine to another.
One approach for addressing machine independence, known as “pointer swizzling,” employs two completely different formats for representing references: a machine-dependent runtime format using pointers for references in main memory, and a platform invariant format for encoding references in secondary storage. When the reference is written to secondary storage, machine pointers are converted into a machine-independent symbol such as a string or a serial number. When the reference is read back into main memory from secondary storage, the symbol is unswizzled and converted back into a machine pointer. Swizzling is also referred to as “serialization” and “pickling.”
The swizzling and the unswizzling operations, however, are computationally expensive, requiring many memory accesses into an auxiliary symbol table, typically implemented by a hash table or binary tree stored in memory. Thus, frequent storage and retrieval of program state into and out of secondary storage can be responsible for a significant drain on system performance. Furthermore, the computational expense of swizzling makes symbols even more impractical as a run-time reference format because conversion between machine pointers occurs much more frequently.